KR100784654B1 - PCM modem adaptation system utilizing a silence period - Google Patents

Abstract

In a full duplex PCM modem system (FIG. 2), robust measurement of communication channel 67 where digital modem 26 transmitter is silenced 54 during generation of a training signal by analog modem 12. A method and apparatus are provided, wherein silencing the transmitter of the digital modem eliminates problems associated with echoing during channel measurement in the digital modem. Thus, the method eliminates echoes during channel estimation and removes any dependence on the performance of the echo canceller normally used.

Full Duplex PCM Modem System, Robust Measurement, Echo Canceller, Error Reduction Device, Digital Modem

Description

PCM modem adaptation system utilizing a silence period

The present invention is particularly in the field of telecommunications to a full duplex PCM modem system in which upstream channel damages or changes are measured during periods of silence within a regular rate renegotiation sequence. It is about.

In a full duplex modem system interconnecting analog and digital modems, after the initial startup procedure and when both modems start transmitting data, time may come to detect or detect the need for the digital modem to update the analog modem parameters. . This is done by examining the upstream channel again to see if the error rate has increased. Since the initial startup is complete, the analog modem transmission is locked to network timing through a loopback timing scheme. This lock to network timing is important because loopback timing is an essential requirement to accurately estimate the channel.

To obtain loopback timing, the modems must be in full duplex mode. However, echo is a problem at this point. When in full duplex mode, the modem transmitters are keyed on and an echo path from the transmitter of the modem to the receiver of the modem where channel probing occurs, through digital-to-analog and analog-to-digital converters. exist. Echo cancellation in digital modems can be very effective, but never 100% effective.

More specifically, it is important that a series of parameters can be generated in the digital modem, which the analog modem used to reconstruct itself taking into account channel variations. What should be measured in the digital modem are these channel variations or impairments. In some cases, pre-equalization parameters, transmit constellation and transmit power in an analog modem may need to be changed to accommodate changes in the upstream communication channel. These changes are made on the analog side in order to maximize the performance of the analog modem, given the channel variations sensed on the digital side.

In full duplex PCM modem systems, echo cancellation is used in both analog and digital modems to prevent echoes generated by modem transmitters from interfering with channel measurements or the data transmission itself. Echo cancellation has been effective in the past to rule out various kinds of distortions. However, echo cancellers may not be of sufficient performance to cancel all transmitted energy that is usually reflected back to the modem's receiver as an echo. This becomes particularly unwieldy when measurements are taken to measure channel fluctuations or impairments during full duplex modem operation.

In order for the digital modem to measure upstream channel characteristics, a test or training protocol is sent from the analog modem to the digital modem. The test or training protocol has known characteristics, which are changed by the channel and detected by the digital modem so that the effect of the channel can be ascertained. Upon identifying channel variations or impairments, a set of parameters are sent back to the analog modem to reconfigure or reset the analog modem to optimize the analog modem for the sensed measurement channel characteristics. Typical parameters are mapping parameters, constellation sets and pre-equalization parameters.

Full duplex transmissions are known to generate an echo path from a transmitter to a receiver where channel characteristics are measured, via a digital-to-analog converter in a central office or CO and vice versa through an analog-to-digital converter. As will be appreciated, the inability to properly offset the effects of echo on the received signal results in a false measurement of the channel, so that a sub-optimal set of parameters is sent back from the digital modem to the analog modem for transmission.

Summary of the Invention
Rather than operating digital and analog modems in full duplex mode, in the present invention the transmission from the digital modem in the rate renegotiation sequence is suppressed during the window in which the test or training protocol from the analog side is transmitted. Stopping transmission in the digital modem completely eliminates echo as a factor in the receiver of the digital modem so that test or training protocol signals can be used to accurately measure upstream channels. Note that the silence period is limited, because if it is prolonged the loopback timing will be lost. If loopback timing is lost, the channel cannot be estimated accurately.

With echo-free measurement of channel characteristics, parameters sent to or returned from an analog modem more accurately reflect channel conditions, and thus more accurate of the precompensation elements used in the analog modem. Generate settings. Precompensation generally includes pre-equalization constants, mapping parameters for setting the output power of the analog modem, and setting of the transmission constellation for optimizing the constellation for the transmission of data.

It will be appreciated that with an optimal set of parameters, communication is more robust and the speed at which the system can operate is increased. If this is not the case, the speed is slowed down or throttled back, as is the case with high error rates caused by changing channel conditions.

In summary, in a full duplex PCM modem system, a method and apparatus for robust measurement of a communication channel in which a transmitter of a digital modem is silenced during generation of training signals by an analog modem is provided, wherein the silence of the transmitter of the digital modem is a digital modem. Eliminate problems associated with echoes during channel measurements at. Thus, the method eliminates echoes during channel estimation and removes any dependencies on the performance of the normally used echo canceller.

These and other drawings will be better understood in conjunction with the detailed description associated with the drawings.

1 is a block diagram of a full duplex PCM modem system including the use of an analog modem and a digital modem, illustrating the echo path from the transmitter of the digital modem to the receiver of the digital modem through the central station.

2 illustrates the transmission of a training sequence from an analog modem to a digital modem during rate renegotiation, and further illustrates the suspension of transmission from the transmitter portion of the digital modem during an established time window for the reception of a test or training sequence. Block diagram of the full duplex PCM modem system of FIG. 1, completely eliminating echoes for accurate channel estimation.

details
Referring to FIG. 1, the PCM modem system 10 includes an analog modem 12 connected to a central station 18 by a hybrid circuit 14 via a local analog loop 15. It should be noted that analog-to-digital converters in similar modems usually exist in the receive path and digital-to-analog converters usually exist in the transmit path. In the central station there is a hybrid circuit 20 which converts analog data into a digital stream using an analog-to-digital converter 24 in the upstream direction illustrated by arrow 22. In particular, it is this information that is applied to the receiver of the digital modem 26 to allow parameter generation and channel measurements illustrated in (28).

Generally, the transmitter of the digital modem 26 continuously transmits an output signal which is connected via the digital-to-analog converter 30 and from there to the circuit 20 in the central station 18. The signal from the central station is applied to the receiver of analog modem 12 via local analog loop 15 and circuit 14 in the downstream direction illustrated by arrow 32. The purpose of transmitting the parameters P1, P2 and P3 is to configure or set the precompensation circuits in the analog modem used to optimize the output of the analog modem for the channel. Among others, parameters P1, P2 and P3 related to the setting of the pre-equalizer, transmission constellations and mapping parameters are used to control the output transmit power of the analog modem.

As can be seen, the echo path 34, illustrated by the dashed lines, exists through the hybrid from the transmitter of the analog modem to its receiver so that the transmitted energy flows back to the receiver. The echo canceller illustrated in (36) was used to cancel the effect of the echoes on the analog side.

A similar type of echo cancellation was used in the digital modems illustrated at 40. Some digital modem echo cancellers consider the effects of digital impairments, downstream and upstream to improve the performance of echo cancellation. The echo canceller is connected between the transmitter and the receiver to cancel the effects of the echo, and the echo path is illustrated by dashed line 42. This echo path travels from the transmitting side to the receiver node 44 via the digital-to-analog converter, through the hybrid 20 and through the analog-to-digital converter. As usual, the echo canceller taps off the signals from the transmitting side and provides an out-of-phase signal to the node 44 to cancel the effects of the echo.

The problem with such a scheme is that some residual echo remains. Invariably, unnecessary components are returned to the receiver of the digital modem, so that channel measurements made at unit 28 are often corrupted along with the residual echo signal. As mentioned above, the result is the generation of parameters P1, P2 and P3 which are not very accurate due to non-optimal channel estimations. When these parameters are prone to error, the analog modem is not set up properly. As a result, the data transfer rate can be lowered and the throughput can be lowered.

It will be appreciated that the provision of the parameters P1, P2 and P3 takes place in a so-called rate renegotiation sequence which is set for the various precompensation components of the analog modem. Usually, P1, P2 and P3 are established during the procedure in which a test or training sequence is sent from the analog modem to the digital modem and the channel measurements are made based on this a priori signal stream. In this renegotiation sequence, the setting errors of the parameters P1, P2 and P3 result from an incorrect measurement of the channel.

With reference to FIG. 2, the aforementioned test or training sequence is set to occur in the window indicated by shading 52 to be illustrated at 50 and to be within a predetermined portion of the data stream from the analog modem to the digital modem. The location of this window is recognized by the digital modem and unit 54 is used to key off the transmitter of the digital modem during this window. As a result, the echo path 42 disappears during the test or training sequence window. Thus, measurements of the channel made at node 44 may be made without echo considerations.

Thus, as shown by echo canceller 40, an echo canceller can be used between the transmitting side and the receiving side, but the quality of this echo canceller can never be such that the echo is completely removed during the training sequence.

In operation, the system is switched to the start mode illustrated at 60. This fact is detected by unit 62, which is used to activate unit 64 to key off the transmitter of the digital modem at a suitable time and thus to provide a silent period.

Software in the digital modem causes the modem to measure the receive channel illustrated at 28 during this silent period and generate new parameter sets P1, P2 and P3 at 64, and the transmitter of the digital modem indicated by arrow 67 Key on and then transmit the new parameters 66 to the analog modem.

"Digital and analog modems using pulse code modulation for data transmission, filed September 3, 1999, which transfers these signals to the digital modem by the analog modem as well as the test and training protocol. (Method and Apparatus for a Startup Procedure for Digital and Analog Modems Utilizing Pulse Code Modulation for Data Transmission) is described in US Patent Application No. 09 / 390,106.                 

Although the present invention has been described in connection with the silence of the transmitter of a digital modem to allow channel measurements to be made when in full duplex mode, the silence system of the present invention may be advantageously used in analog modems. This allows the analog modem to detect downstream channels free of echo interference.

One difference, however, is that loop backed timing is not lost if the analog modem transmitter is interrupted. This is because the analog modem can track the network timing through the received signal from the digital modem. When operated, the analog modem can continue locked transmission at network timing.

Thus, the echo cancellation discussed in connection with the digital modem can be reflected in the analog modem if required.

Having described various embodiments of the present invention and various modifications to the embodiments, it should be apparent to those skilled in the art that the above embodiments are merely exemplary and do not limit the present invention. Many modifications and other embodiments are within the scope of common sense of those skilled in the art and are considered to be within the scope of the invention, which is limited only by the appended claims and their equivalents.

Claims (11)

In a full duplex PCM modem system, a method for removing effects of echo on channel estimation in a modem due to an echo path between a modem's transmitter and receiver, the method comprising: Keying off the transmitter for a period during which channel estimation measurements are to be made. In a full duplex PCM modem system with analog and digital modems interconnected via a network channel, a method of reducing errors in channel estimation in the digital modem, the method comprising: Generating a sequence of training signals in the analog modem; Transmitting the training signals to the digital modem via a network channel; And Keying off the transmitter of the digital modem during the period in which the training signals are received at the digital modem, whereby echoes typically associated with signals from the transmitter of the digital modem to the receiver are canceled, thereby receiving the receiver of the digital modem. And the transmitter key off step allowing channel estimates in to be free of error components due to echo. The method of claim 2, And the modems are locked to network timing and the transmitter is keyed back on before network timing is lost in the analog modem. The method of claim 2, The digital modem is switched to start-up mode, Detecting when the digital modem will switch to the start mode, wherein the key off of the transmitter occurs upon detecting switching from the digital modem to a start mode. The method of claim 2, Measuring the channel during the period that the transmitter is keyed off, thereby allowing echo-free channel estimation. The method of claim 5, Determining new parameters in response to the channel estimate. delete In a full duplex PCM modem system with analog and digital modems connected over a network channel, the apparatus for reducing errors in channel estimation in the digital modem, A transmitter in the digital modem; And And a unit for causing transmission from the transmitter to be interrupted during the channel estimation, thereby eliminating effects of echo on the channel estimation. The method of claim 8, And the unit comprises a keying circuit to turn on and off the transmitter. The method of claim 9, Further comprising network timing applied to the digital modem, wherein the transmitter is operated before a lock to the network timing is lost by the analog modem. The method of claim 9, Circuitry for placing the digital modem in a start mode and a detector having a predetermined output when the digital modem is placed in the start mode and the channel estimation is about to begin, the unit responding to the output of the detector Error reduction device.

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